Configuring ISDN BRI Voice Ports

Cisco routers support several types of ISDN voice interface cards such as VIC-2BRI-S/T-TE and VIC-2BRI-NT/TE, which can provide connectivity to a PBX or PSTN. A benefit of using the ISDN BRI VIC rather than the analog VIC modules is the additional calling information that is passed.

Up to four calls are supported when the VIC-2BRI is installed in the NM-2V module. The BRI VIC needs to be installed in Slot 0 of the NM-2V for both ports to be active. If an additional VIC is installed in the second slot of the NM-2V, the second port on the first VIC will be disabled. This is based on the two ports of the BRI VIC requiring four DSPs. The VIC interface modules support both ISDN network and user-side configurations. The following are the steps necessary to configure ISDN BRI to a PBX:

  1. Enter global configuration mode:

    router# configure terminal
  2. Set the global ISDN switch type. The only NT supported type is basic-net or basic-qsig:

    router(config)# isdn switch-type switch-type
  3. Set the ISDN BRI interface slot and port:

    router(config)# interface bri slot|port
  4. Ensure that no IP address is configured for the ISDN interface (voice only):

    router(config-if)# no ip address
  5. Specify incoming voice calls over ISDN:

    router(config-if)# isdn incoming-voice voice
  6. Shut down the interface. Configure the physical layer type. Enable interface with no shutdown:

    • Enter user to configure the port as TE and to function as a clock slave. This is the default.

    • Enter network to configure the port as NT and to function as a clock master.

    router(config-if)# shutdown
    router(config-if)# isdn layer1-emulate {user | network}
    router(config-if)# no shutdown
  7. Turn on or off the power supplied from an NT-configured port to a TE device:

    router(config-if)# [no] line-power
  8. Configure the Layer 2 and Layer 3 port protocol:

    router(config-if)# isdn protocol-emulate {user | network}
  9. Exit global configuration mode:

    router(config)# end

Figure 1 illustrates a scenario with ISDN BRI connectivity to a PBX and PSTN. A partial configuration lists the commands pertaining to the PSTN and PBX interfaces.

Figure 1: ISDN BRI PBX and PBX Scenario
(Partial Cisco 1760 configuration)
hostname 1760
isdn switch-type basic-net3
interface BRI 1/0
no shutdown
description connected to PBX
no ip address
isdn switch-type basic-net3
isdn incoming-voice voice
isdn layer1-emulate user
no shutdown
isdn protocol-emulate user
interface BRI 2/0
no shutdown
description connected to PSTN
no ip address
isdn switch-type basic-net3
isdn incoming-voice voice
isdn overlap-receiving
isdn layer1-emulate network
no shutdown
isdn protocol-emulate network

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